A standard NTSC television scans 525 lines per frame in the form of two sequential fields of 2621/2 lines each. The lines of each field interlace with the lines of adjacent fields and the eye integrates these interlaced lines to reduce the effect of a 30 Hz frame-rate flicker. The horizontal line structure is still visible under certain circumstances, however, and is particularly visible on large-screen television displays viewed from a relatively close distance. The problem is made even more severe by the ultra-large pictures formed by projection-type television displays. The advantages of such ultra-large pictures in providing the illusion of surrounding the viewer are reduced by the need for the viewer to remain sufficiently far from the display to integrate the line structure.
A compatible high-definition television system is described in U.S. patent application Ser. No. 288,753 filed July 31, 1981, now U.S. Pat. No. 4,429,327 in the names of C. B. Oakley and R. A. Dischert which issued Jan. 31, 1984, as U.S. Pat. No. 4,429,327. In this system, the visibility of the horizontal line structure is reduced in a manner compatible with standef NTSC (or PAL) television receivers by using a camera which generates two lines for every standard line (for example, 1,050 lines-per-frame rather than 525), forming separate signals related to the sums and differences of pixels on adjacent raster lines, and transmitting as a compatible signal the sum signal, together with the difference signal, which may be transmitted separately or concealed within a composite color signal. This arrangement increases the vertical resolution by increasing the number of horizontal lines, which makes it possible to view an ultra-large picture from a closer distance without discerning the line structure. With this system, the vertical luminance resolution becomes about 970 lines, while the horizontal resolution, which is established by the luminance bandwidth, remains at about 330 television lines per picture height. The horizontal resolution thus becomes the limiting factor in the distance between the viewer and an ultra-large display, once the line structure becomes invisible.
Another compatible high-definition television system is described in U.S. patent application Ser. No. 687,462 filed Dec. 28, 1984, which is a continuation-in-part of U.S. patent application Ser. No. 352,001 filed Feb. 24, 1982, in the name of R. N. Hurst. In this system, the scanning spot in the camera is wobbled to double the resolution in both the horizontal and vertical directions of the high-definition display. The wider bandwidth signal which is transmitted is compatible with standard television receivers, the effect of the narrow bandwidth of such receives is to average the values of adjacent pixels in both horizontal and vertical directions. In the high-definition wider bandwidth television receiver, the scanning spot is synchronized to wobble in accordance with the wobble that was introduced by the camera. According to Hurst, the spot is wobbled at a rate equal to an odd integer multiple of one-half of the horizontal scan frequency so that a complete high-definition raster of pixels is traced out over four successive fields.
A disadvantage of spot wobbling at an odd integrer multiple of one-half the horizontal line rate is that certain scanning artifacts on the television display may become visible and be objectionable to the viewer. When the spot is wobbled at an odd integer multiple of one-half the horizontal rate the wobble phase on successive lines of a given field differ by 180.degree.. Therefore, the scanning line structure will display a visible high-frequency modulation of the space between adjacent lines of the same field giving the picture an appearance of an array of black dots superimposed over the image. The interstitial lines of one field will not overlay on the black spaces of the previous field and thus the array of black dots will appear to move either vertically, horizontally, or along 45.degree. lines in any of four directions.
If the spot wobbling frequency is chosen to be an even integer multiple of one-half of the line rate a herringbone pattern of lines is produced on the display. In this case the interstitial scan lines of one field do overlay the black lines of the previous field, however, not all of the picture elements of a high-definition television raster will be scanned resulting in a lack of full resolution on the display.